Fixes for OpenCL global variables to be in __constant address space

1. nbnxn_consts.h contains constants shared between the OpenCL device
code and the host code. Added a macro CONSTANT_ADDRESS_SPACE which is
__constant for the devce, const for host. Name can be improved
2. Moved #define __IN_OPENCL_KERNEL__ from nbnxn_ocl_kernel_utils.clh to
nbnxn_ocl_kernels.cl